I personally use them right away. Because I can use the 50% moogle right away, but the possibility of pulling a fourth copy is super low. It can happen, of course, but the game won't last forever either, so I'd rather take my chances with the stuff the game throws at me now than hoarding for a possible outcome that may never happen

I used to instantly merge them for the Moogles, since the 7* era just started and I suddenly needed TMRs for units that were low on my priority list.

Right now I'm just letting them sit there. We've been getting so many free moogles from the Fan Festa that I can be patient. Sitting on 250% of Moogles and could easily double that from 5* merges (probably more, haven't checked in a while).

Look at it this way: A benched 5* takes just as much space in your inventory as a benched 50% moogle.

Atk is not easy to get while Def can be easily acquired with LMS.
I'm still struggling to reach and I have a full TDH set + DKC's STMR (only STMR in my build).
Spr is also very hard to reach if you don't have a few chosen 7* units.

Look at it this way: A benched 5* takes just as much space in your inventory as a benched 50% moogle.

Click to expand...

Yeah, but it still depends on if you bench your moogles or use them immediately

I guess this really is the core question in this case. If you still have use for moogles all the time (like me), then it might be worth getting them immediately, considering that you might never get the fourth copy, which is more probable than otherwise.
But if you don't use the moogles asap and already have a stash of them, then you can as well save those 5* bases for the low possibility of a fourth copy

small rant: I haven't been blessed with any 4th copy, what more for 5th one
I think I'm one of the select few that have been actively playing since the beginning with zero STMRs (except for Katy Perry's of course).

Well at 3rd copy I just merge and get the 50% moogle to get it over with.

If I have 4 copies and the stmr is not too great I wait for 5th. Like LM fina.

Reason being, at some point you will have so many 7* the expert missions will be completed anyhow.

Every 7* awakening is 3mil gil so after awakening 74 I'm pretty broke. Not much reason to hold on to 3rd copies. Expert mission asks for 100 7*, I'm sure I'll get there even if I didn't awaken all the 4 copies units I have.